1. Field of the Invention
The present invention relates to a torque-limiting coupling device having two coaxial, generally cylindrical co-acting surfaces on two co-acting parts in the form of a cylindrical sleeve and a cylindrical shaft, respectively. The sleeve is in frictional engagement with the shaft for transmission of torque up to a limit that corresponds to the frictional engagement and at which the sleeve begins to rotate relative to the shaft. The device further includes a pump mechanism which, upon relative rotation between the sleeve and the shaft, is driven to pump liquid to an interface between the co-acting surfaces, with channels being provided to carry away liquid from the interface so as to restore the frictional grip after the torque limit has been exceeded.
2. Description of the Related Art
A coupling device of the kind in question is disclosed in WO 90/00231, which corresponds with U.S. Pat. No. 5,069,320. This known device generally functions well. Devices of this kind are often used in steel rolling mills between a drive motor, for instance an electric motor, and a roll. The power transferred may be in the order of 20,000 kW. Idling costs in respect of such a rolling mill may lie in the order of up to 100,000 Swedish kroners (SEK)/hour.
In the case of the areas of use concerned, the device is triggered with a relatively low frequency, for instance a frequency in the region of once every five years to up to 300 times annually. A typical activating frequency is twenty times per annum.
Basically, the coupling device comprises two axially, generally cylindrical interacting surfaces on two interacting parts in the form of a cylindrical sleeve and a cylindrical shaft respectively, wherein the sleeve is in a torque-transmitting frictional contact with the shaft for transmission of torque up to a corresponding limit, after which the sleeve begins to slide or slip relative to the shaft. The frictional engagement can be adjusted to a selective level with the aid of some suitable technique. For instance, the sleeve may include a concentric ring-chamber that can be placed under pressure. The coupling device includes one or more pumps that function to pump liquid from a liquid store to the gap between the interacting surfaces, such that the liquid will form a hydrostatic layer together with said interacting surfaces. The pumps are intended to be driven by relative rotation between said parts. As a result of pumping liquid in between the mutually co-acting surfaces, said surfaces are able to slide relative to one another as soon as the set torque is exceeded. This enables damage to the coupling device and to the motor or rolling mill to be avoided. A torque-limiting coupling device of this known kind need only rotate through barely one revolution in order to generate an hydrostatic layer for which the torque is reduced to a level close to zero.
When the device is triggered, it is necessary to stop the drive completely. The pumps in the coupling device will then stop pumping liquid (oil) in between the mutually interacting surfaces. This enables the liquid to drain away through channels, wherewith the frictional engagement between said mutually interacting surfaces is re-established within the space of about 1 minute. The torque limit can be chosen within a wide range, and can be set with a high degree of accuracy, for instance ±10% from a desired value. The coupling device also has compact dimensions.
Problems associated with this known device reside in the possibility of all or part of the liquid/oil contained in the device leaking away before the device is triggered, or of by a change in the properties of the oil with time, contamination of the oil with particles that can be formed, for instance, in conjunction with triggering the device and that clog valves, filters, and disturb or interfere with the function of the device, and so on.
There is therefore a certain risk that this known device will malfunction when triggered. Malfunctioning of the device would mean that the mutually interacting surfaces would not be separated sufficiently and that insufficient oil would be pressed in between the interacting surfaces. The malfunction may then cause a very high torque to be transferred via the device in spite of the torque-limiting mechanism, therewith resulting in damage to the motor and driven equipment for instance, and also such as to cause serious damage to the coupling device itself. Particularly with a view to the consequence of such a malfunction (compare the idling cost), the type of device concerned has not found particularly wide use in practice, despite the ability of the device to enable automatic and fast resetting with normal triggering of the device, and thereby a fast return to plant operation after having removed or rectified the triggering cause.